1. Field
The present disclosure relates to a semiconductor device such as a power semiconductor module.
2. Related Art
In recent years, in response to a demand for energy saving, expanding applications of power semiconductor modules have been in progress. An explanation is made about an example of this power semiconductor module. FIG. 8 is a substantial part circuit diagram of a power semiconductor module 500. A circuit of this power semiconductor module 500 is composed of a converter part 81, a brake part 82, and an inverter part 83.
The converter part 81 is constituted of three-phases of a U-phase, a V-phase, and a W-phase. Each phase is configured of upper and lower arms and each arm is provided with a diode 84. A brake part 82 is provided with a diode 85 and an IGBT (insulated gate bipolar transistor) 86. In the brake part 82, the diode 85 and the IGBT 86 are connected in parallel.
The inverter part 83 is constituted of three-phases of a U-phase, a V-phase, and a W-phase, and each phase is configured of upper and lower arms. One arm of the inverter part 83 is provided with an IGBT 87 and a FWD (freewheeling diode) 88, which is a diode. The IGBT 87 and the FWD 88 are connected in reverse parallel. In the inverter part 83, single ones of or plural ones of the IGBT(s) 87 and the FWD(s) 88 may be connected in parallel. The FWD 88 used in the inverter part 83 has a reverse recovery mode. A high reverse recovery tolerance is required because a breakdown tends to occur in this reverse recovery mode.
FIG. 9 is a substantial part cross-sectional view of an FWD 88 in the past. The cross-sectional view of FIG. 9 is an enlarged view of an edge termination structure and the vicinity thereof, and illustrates a structure similar to one, for example, illustrated in FIG. 1 of Patent Literature 4. In FIG. 9, the FWD 88 is provided with a p-anode region 54 and an anode edge part region 55 that are disposed on an n-drift region 52 formed in an n-semiconductor substrate 51, and p-guard ring regions 58 and a p-stopper region 59 that are disposed on an n−-region 53 that is an extending part of the n-drift region 52. In addition, the FWD88 is provided with an anode electrode 60 disposed on the p-anode region 54 and the anode edge part region 55.
In addition, the FWD 88 is provided with insulating films 61 disposed on the p-guard ring regions 58 and the p-stopper region 59. Moreover, the FWD 88 is provided with guard ring electrodes 62 that are electrically connected to the corresponding p-guard ring regions 58, and a stop electrode 63 that is electrically connected to the p-stopper region 59. A diffusion depth and an impurity density in the anode edge part region 55 are the same as a diffusion depth and an impurity density in the p-guard ring region 58, respectively.
Furthermore, the FWD 88 is provided with an n-cathode region 65 disposed below the n-drift region 52, and a cathode electrode 66 that is electrically connected to the n-cathode region 65. In the FWD 88, a region where the p-anode region 54 is formed is an active part 64; and a region where the p-guard ring regions 58, the p-stopper regions 59, and the insulating films 61 are formed is an edge termination structure 67. Incidentally, a symbol 52 in the drawing represents an n-drift region; and a symbol 53 represents n−-region that is the extending part of the n-drift region 52. These regions 52, 53 may be referred to collectively and simply as drift regions.
Next, a circuit operation of the three-phase inverter circuit (the inverter part 83) when an inductance such as an un-illustrated motor is connected to the inverter circuit (the inverter part 83) as a load. Here, paying attention to the U phase and the V phase, an explanation is made about a case where the IGBT 87 of the lower arm of the W phase and the IGBT 87 of the upper arm of the U phase are both turned on, which causes an electric current to flow through the motor, thereby rotating the motor. Although an electric current flows through the lower arm or the upper arm of the W phase in reality, such electric current is omitted for the sake of convenience, and a case where a single phase inverter of the U phase or the V phase are connected to the motor is explained.
The IGBT 87 of the upper arm of the U phase and the IGBT 87 of the lower arm of the W phase are repeatedly on and off. A longer on-period allows the electric current flowing through the motor to be increased; and a longer off-period allows the electric current flowing through the motor to be decreased. With these increase and decrease of this electric current, a torque and a rotation speed of the motor are controlled. In the following, a time during which the FWD 88 is on is called a time of electrifying; and a time at which the FWD 88 is shifted from the on-state to the off-state is called a time of reverse recovery.
FIG. 10 is an explanatory view illustrating a voltage-current waveform of the FWD 88 and the IGBT 87. Using FIG. 10, the operations are explained for each section thereof. (1) A-section is in the state where the IGBT 87 is on and the electric current is supplied to the motor. In the state where the IGBT 87 is on and the electric current is supplied to the motor, no electric current flows through the FWD 88. (2) B-section is in the state where the IGBT 87 is off. In this case, the electric current that has flown through the motor loses its destination, and thus flows into the inverter part 83 by way of the arm of the FWD 88. This electric current is called a reflux electric current, which is a forward electric current in relation to the FWD 88. When the forward electric current flows in the FWD 88 is the “time of electrifying”. (3) C-section is in the state where the IGBT 87 is on again. The electric current due to the IGBT 87 being on is a reverse electric current in relation to the electric current flowing through the motor and the FWD 88 connected to this IGBT 87 in series. The reverse electric current flowing in the FWD 88 comes into halt at the stage where the FWD 88 is reverse recovered, and all the current flows into the motor. This series of the operations are repeated, so that the electric current flowing in the motor is controlled. The reverse electric current flowing in the FWD 88 is the reverse recovery electric current, and a time during which this reverse recovery electric current flows is the “time of reverse recovery”.
FIG. 11 illustrates views for explaining behavior of holes flowing in the FWD 88, wherein (a) is a view at the time of electrifying and (b) is a view at the time of reverse recovery. In FIG. 11, the behavior of the holes flowing in the FWD 88, especially, the behavior in an area from the active part 64 through the edge termination structure 67 is illustrated. Incidentally, while accumulation of hole-electron pairs is caused in the n-drift region 52 below the active part 64 at the time of electrifying, the illustration is omitted.
At the time of electrifying in FIG. 11, part (a), the holes are injected from the p-anode region 54 into the n-drift region 52 and the n−-region 53 as the extending part of the n-drift region 52, (see a symbol 71 in FIG. 11, part (a)). The electrons are injected from the n-cathode region 65 into the n-drift region 52 and the n−-region 53 so as to neutralize the holes. As a result, a forward electric current flows in the n-drift region 52 and the n−-region 53 under a state where the excessive holes and the excessive electrons exist (under a state with hole-electron pairs accumulated). Such a state where the forward electric current flows with those excessive holes and electrons existed is called a conductivity modulation, where resistances in the n-drift region 52 and the n−-region 53 are greatly reduced. Namely, at the time of electrifying, the excessive holes and electrons are accumulated in the n-drift region 52 and the n−-region 53.
In the reverse recovery of FIG. 11, part (b), the IGBT 87 turns on again, and thus the FWD 88 is shifted into the reverse recovery process where the reverse recovery electric current flows. At the time of this reverse recovery, regarding the holes and the electrons which are accumulated in the n-drift region 52 and the n−-region 53, withdrawal of the holes is carried out in the p-anode region 54 and the anode edge part region 55 (see a symbol 73 in FIG. 11, part (b)), and thus the electrons are withdrawn to the n-cathode region 65, thereby producing the reverse recovery electric current. The reverse recovery electric current is lost at a stage where the excessive holes and electrons come into non-existence in the n-drift region 52 and the n−-region 53, and thus the FWD 88 becomes in the off state.
A pn-junction of the anode edge part region 55 and the n-drift region 52 has a shape of convex in the depth direction. Therefore, electric current tends to concentrate thereto, rather than a flat bottom portion of the p-anode region 54. In addition, the holes accumulated in the n−-region 53 below the edge termination structure 67 flow through the anode edge part region 55 in a concentrated manner at the time of reverse recovery, thereby causing breakdown of the FWD 88.
As a method to make it difficult for the electric current to concentrate into the anode edge part region 55, there is a method of taking an extension structure 68 where a resistance region 56 is provided between the p-anode region 54 and the p-guard ring region 58, which is a part of the edge termination structure 67 (see FIG. 1 of Patent Literature 1 below, for example). In addition, a method has also been disclosed where a lifetime is locally shortened in the p-anode region 54, the edge termination structure 67, and a junction and its vicinity of the n-drift region 52 and the n−-region 53 as the extending part thereof (see FIG. 1 of Patent Literature 2 below, for example).
FIG. 12 is a substantial part cross-sectional view of the FWD 88 having the extension structure 68. A difference between the FWD 88 illustrated in FIG. 12 and the FWD 88 illustrated in FIG. 9 exists where the resistance region 56 into which the p-anode region 54 is extended in a direction toward the outer circumference thereof is provided between the p-anode region 54 and the p-guard ring regions 58, which are the edge termination structure 67. The anode electrode 60 is kept away from the upper surface of the resistance region 56 with an insulating film intervening there between. By applying the extension structure 68, the electric current concentration to an extending edge part region 57 at the time of reverse recovery is alleviated, so that the breakdown of the FWD 88 is prevented.
In addition, Patent Literature 3 below describes a deep intermediate layer that has a lower density than the guard ring and a higher density than an anode lower density layer, for example, in FIG. 3. However, this intermediate layer is not connected to the guard ring. Moreover, a contact edge part is in contact with the lower density layer. Furthermore, Patent Literature 4 below describes an effect due to resistance in the same structure as that in the example in the past, for example, in FIG. 1 and FIG. 5. In addition, Patent Literature 5 below describes a configuration where the contact edge part is covered by a p-ring, which is suggested to have a lower density and deeper diffusion depth than the anode layer.